Clearwater County Frost Dates and Growing Season

NOAA 1991-2020 normals for Headquarters, 12.5 km from Clearwater County, elevation 3200 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Headquarters. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

NOAA does not state freeze dates for this station.

Median at 28°F: last May 17, first September 20

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Jun 7	May 30	May 25	May 21	May 17	May 14	May 11	May 8	May 5
24°F	May 14	May 10	May 6	May 4	May 1	Apr 30	Apr 27	Apr 22	Apr 16
20°F	Apr 30	Apr 24	Apr 18	Apr 14	Apr 10	Apr 6	Apr 1	Mar 27	Mar 20
16°F	Apr 9	Apr 1	Mar 27	Mar 21	Mar 17	Mar 13	Mar 9	Mar 5	Feb 27

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Sep 1	Sep 7	Sep 12	Sep 16	Sep 20	Sep 23	Sep 27	Oct 1	Oct 5
24°F	Sep 19	Sep 26	Sep 29	Oct 3	Oct 6	Oct 10	Oct 13	Oct 17	Oct 23
20°F	Oct 4	Oct 11	Oct 17	Oct 22	Oct 25	Oct 30	Nov 2	Nov 9	Nov 19
16°F	Oct 21	Oct 28	Nov 2	Nov 8	Nov 13	Nov 19	Nov 24	Nov 29	Dec 6

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	66	58	52	45	41	37	33	29	24
32°F	108	98	92	86	81	76	70	64	55
28°F	145	137	132	127	122	117	111	104	94
24°F	178	171	166	161	156	152	148	141	132
20°F	227	216	208	202	196	191	186	179	169
16°F	271	259	251	243	237	231	224	217	206

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder: 100% of years
32°F or colder: 100% of years
28°F or colder: 100% of years
24°F or colder: 100% of years
20°F or colder: 100% of years
16°F or colder: 100% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F: 8,243
Cooling degree days, base 65°F: 73
Growing degree days, base 40°F: 2,644
Growing degree days, base 45°F: 1,794
Growing degree days, base 50°F: 1,114
Growing degree days, base 55°F: 598
Growing degree days, base 60°F: 254

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	1,186	0	0
February	984	0	0
March	952	0	0
April	766	0	7
May	511	1	73
June	306	6	181
July	133	41	375
August	161	23	331
September	373	2	133
October	719	0	13
November	956	0	0
December	1,196	0	0

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 12.5 km from Clearwater County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00104150 (Headquarters). Public domain.